AGR 3102 Principles of Weed Science Herbicide Muhammad Saiful Ahmad Hamdani

Unit 7 – Topics Covered Herbicides: Selectivity Resistance

Herbicide Selectivity Herbicide selectivity is the basis for successful chemical weed management in crops. Selectivity is accomplished primarily by 3 methods: Selectivity by Placement Selectivity avoiding or minimizing contact between the herbicide and the desired crop is called selectivity by placement. Mainly for non-selective herbicides. Example: apply glyphosate directly to the weed, not exposing to the crop. Suitable esp. for big and tall crops i.e. orchard, plantation, landscape.

Selectivity by Differences in Crop and Weed Growth Stage Selectivity as a result of some morphological and physiological between crop and weed. Selectivity mainly for non-selective herbicides. When crop is older than the weed, crop will have an advantage on things such as: - thicker cuticle and leaf wax: less herbicide penetration. - more leaves: many angles - difficult to get a good coverage; insufficient translocation of herbicide. - higher growing point (meristem): growing point is protected, less likely for foliar herbicides to reach the growing point. - deeper root: less absorption: insufficient translocation.

Q1: Why selective herbicides; i. e Q1: Why selective herbicides; i.e. ACCase herbicides (graminicides) only kill grass weeds but not broadleaf weeds? Q2: And why ACCase herbicides don’t have effect on grass crops too????? Q3: How about PGRs, where grass weeds/crops are not affected, but broadleaf crops/weeds are? True Selectivity Selectivity because of the differences in the biochemical process in plants. For selective herbicides. A1: Broadleaf weeds and crops/plants are not affected by ACCase herbicides because their ACCase is less sensitive to these herbicides.

A2: Grass crops (corn, rice, wheat…) are also not affected by ACCase herbicides because they can rapidly metabolise these herbicides into inactive product (difference in metabolism rate between grass crops and grass weeds). A3: As for the PGRs, broadleaf plants are more sensitive to the changes in the auxin level than the grass plants.

Q4: How about dinitroanilines from seedling growth inhibitors Q4: How about dinitroanilines from seedling growth inhibitors? They are not metabolized by plants. A4: Difference in absorption of dinitroaniline herbicides between tolerance crops and susceptible weeds. And of course, herbicide placement plays an important part too…

Evolution of Herbicide Resistance in Weed Species Herbicides have revolutionized weed control practices all over the world. Farmers preferred herbicides over cultural and mechanical control practices because of time and cost efficiency, easy to apply, and, fast + effective weed control. However, good things won’t last long..... Repeated use and strong reliance on herbicides led to evolution of herbicide resistance in weed populations, especially for herbicides with a single MOA.

What is herbicide resistance (HR)??? The inherited ability of a plant to survive and reproduce following exposure to a dose of herbicide that would normally inhibit and kill wild type individuals of the same population. What is herbicide susceptibility??? The degree to which a plant is subject to injury or killed by a particular herbicide. What is herbicide tolerance??? The inherited ability of a species to survive and reproduce following a herbicide treatment.

So is there any difference between HR and herbicide tolerance??? There is. No selection to make the plants tolerant; those plants simply possess a natural tolerance. Resistant (R) plants were originally susceptible (S), but through continuous exposure to herbicide, they become R. How plants evolved resistance??? - continuous selection pressure from the HERBICIDE, naturally or induced via genetic engineering by human.

Only R plants remain: control failure.

Types of Herbicide Resistance Cross resistance Plant is resistant to two or more herbicides having the same MOA. i.e. Lolium rigidum is resistant to diclofop (WSSA Group 1/HRAC Group A; Chemical Family APP; MOA ACCase biosynthesis inhibitors) and clethodim (also WSSA Group 1/HRAC Group A; Chemical Family CHD; MOA ACCase biosynthesis inhibitors).

Multiple resistance Plant is resistant to two or more herbicides having different MOA. i.e. Eleucine indica is resistant to glyphosate (WSSA Group 9/HRAC Group G; Chemical Family Glycines; MOA EPSPS inhibitor) and fluazifop-P-butyl (WSSA Group 1/HRAC Group A; Chemical Family APP; MOA ACCase biosynthesis inhibitors).

Major herbicide resistance mechanism Susceptible weed Target molecule Target molecule 1. Target site mutation Herbicide Resistance weed Target molecule Target molecule

2. Herbicide metabolism Herbicide Herbicide Target molecule Resistant weed

Evolved herbicide resistance patterns

Glyphosate is a very effective herbicide Broad weed spectrum Systemic activity Glyphosate has favorable toxicological and environmental properties So the adoption begins….. What happened not so long after?

Weeds are still winning… IF weeds could talk… “GM crops? It was fun, nice try human…” (Mr. and Mrs. Resistant Weed Species, 1995-now)

Herbicide Resistance Management Divided into 2 types: 1. Prevention and Delaying Resistance Use herbicides wisely Where available, herbicide applications should be based on economic thresholds – control when necessary (critical control period). And don’t cut the rates. Herbicide Rotation Don’t stick to a single herbicide or herbicides with a single MOA. Crop Rotation Can manipulate planting time, spectrum of weed infestation, cultivation techniques, choice of herbicide with different mode of action, different stage and different way of application.

Integrated Weed Control Methods Mechanical weeding eliminates the weed plants before the seed set. Mulching will simultaneously cover weed seeds and stop from germinating. 2. Post-evolution Crop rotation and herbicide rotation will keep the resistant population down. Maximize crop competition (row spacing, proper fertility, optimum planting dates, water management etc.). Ensure clean and certified seed is planted each season. Clean farm machinery is important to avoid movement of seeds from HR weed spp.

We have always had weeds, and….. We always will have weeds, so….. Deal with it…